A police convoy escorting a civilian under a witness protection scheme is attacked, the witness hospitalized and all the police officers killed, except Inspector Lindsay Denton, who organized the operation at very short notice and who only informed DCC Dryden of the route. Ted Hastings asks Steve and Kate to interview her but Kate pulls out, claiming one of the dead officers, Jayne Akers, was a friend - though she has been having an affair with Jayne's husband Rich. Her place is taken by Georgia Trotman, who is troubled that the men are savage with their interrogation, treating Lindsay as a suspect and delving into her finances. Lindsay herself is moved to a missing persons unit but is shown to have a violent streak, attacking a noisy neighbour. Kate follows her to the retirement home where her mother lives and finds she has rung the hospital from a payphone. Georgia and Steve attend at the hospital where somebody is trying to kill the witness and who kills Georgia.

Following the deaths of Georgia and the witness it is Steve who gets a grilling as to the police presence at the hospital whilst Lindsay is accused of attacking her neighbour, which she denies. Steve discovers that a nurse on the witness's ward, Claire Tindall, was threatened into giving the killer access by a man she knows as Joe, but whom she is unable to identify. Lindsay continues to search for vanished teenager Carly Kirk and Kate is drafted in as her assistant - in reality a move to try and get Lindsay to admit her guilt - but Lindsay sees through the ruse. After Kate's cover is blown Lindsay is questioned again and claims that she rang the hospital to ask that the witness exonerate her but she is disbelieved and Hastings arrests her for conspiracy. At her interview she points out that Steve visited Claire unofficially and Hastings has financial problems and a broken marriage that could make him just as culpable but she is nonetheless held in custody. Meanwhile DCC Dryden finds himself in the frame for a driving offence supposedly committed by his wife.

Lindsay is remanded in custody where she is victimized by both staff and prisoners. Meanwhile Steve and Kate discover the identity of the dead witness from blunt inspector Cottan. He was known as Tommy and Cottan believes that Jayne Akers, his liaison officer, was the real mole who betrayed the escort. Kate visits Lindsay, who knows about her affair with Jayne's husband whilst Cottan has similar suspicions. Lindsay's accusations also cause friction between Hastings and Steve, whilst Dryden's driving offence attracts the press so to save himself he strikes a deal naming Lindsay as the officer under investigation. Kate discovers a body, apparently that of the missing Carly, and returns to see Lindsay, who claims her ex-lover Dryden set her up. Kate makes a call which seems to support this whilst Cottan gets evidence to prove Jayne was on the take.

An arrogant Dryden is questioned regarding the traffic offence and is dismissive though Steve lets him know Lindsay has revealed that he knew about the convoy route and aims to pursue it further. Rich Akers is also pulled in after £20,000 of which he claims ignorance is found under the floor boards at his house. Hargreaves is convinced this was the bribe given Jayne to betray the escort and when Steve and Kate examine Jayne's car they discover a tracking device though Hargreaves is not keen for its revelation. Furthermore Cottan and Hargreaves see video evidence suggesting that Tommy had a grievance against Dryden, whose affair with Lindsay is now proved. Lindsay gets a day release from prison to visit her mother, in hospital after a stroke, but on the way back the prison van is hi-jacked and she is abducted by two policemen, Prasad, identified by Carly's parents as her apparent boyfriend, and Cole.